<template>
  <div id="app">
    <div id="demo">
      <div>
        <div>
          <h1>JavaScript</h1>
          <h2>History</h2>
        </div>
      </div>
      <div>
        <div>
          <h3>Creation at Netscape</h3>
          <p>
            The Mosaic web browser was released in 1993. As the first browser
            with a graphical user interface accessible to non-technical people,
            it played a prominent role in the rapid growth of the nascent World
            Wide Web. The lead developers of Mosaic then founded the Netscape
            corporation, which released a more polished browser, Netscape
            Navigator, in 1994. Navigator quickly became the most used browser.
          </p>
          <p>
            During these formative years of the Web, web pages could only be
            static, lacking the capability for dynamic behavior after the page
            was loaded in the browser. There was a desire in the burgeoning web
            development scene to remove this limitation, so in 1995, Netscape
            decided to add a scripting language to Navigator. They pursued two
            routes to achieve this: collaborating with Sun Microsystems to embed
            the Java programming language, while also hiring Brendan Eich to
            embed the Scheme language.
          </p>
          <p>
            Netscape management soon decided that the best option was for Eich
            to devise a new language, with syntax similar to Java and less like
            Scheme or other extant scripting languages. Although the new
            language and its interpreter implementation were officially called
            LiveScript when first shipped as part of a Navigator release in
            September 1995, the name was changed to JavaScript three months late
          </p>
          <p>
            The choice of the JavaScript name has caused confusion, sometimes
            giving the impression that it is a spin-off of Java. Since Java was
            the hot new programming language at the time, this has been
            characterized as a marketing ploy by Netscape to give its own new
            language cachet.
          </p>
          <h3>Adoption by Microsoft</h3>
          <p>
            Microsoft debuted Internet Explorer in 1995, leading to a browser
            war with Netscape. On the JavaScript front, Microsoft
            reverse-engineered the Navigator interpreter to create its own,
            called JScript.
          </p>
          <p>
            JScript was first released in 1996, alongside initial support for
            CSS and extensions to HTML. Each of these implementations was
            noticeably different from their counterparts in Navigator. These
            differences made it difficult for developers to make their websites
            work well in both browsers, leading to widespread use of "best
            viewed in Netscape" and "best viewed in Internet Explorer" logos for
            several years.
          </p>
          <h3>The rise of JScript</h3>
          <p>
            In November 1996, Netscape submitted JavaScript to ECMA
            International, as the starting point for a standard specification
            that all browser vendors could conform to. This led to the official
            release of the first ECMAScript language specification in June 1997.
          </p>
          <p>
            The standards process continued for a few years, with the release of
            ECMAScript 2 in June 1998 and ECMAScript 3 in December 1999. Work on
            ECMAScript 4 began in 2000.
          </p>
          <p>
            Meanwhile, Microsoft gained an increasingly dominant position in the
            browser market. By the early 2000s, Internet Explorer's market share
            reached 95%. This meant that JScript became the de facto standard
            for client-side scripting on the Web.
          </p>
          <p>
            Microsoft initially participated in the standards process and
            implemented some proposals in its JScript language, but eventually
            it stopped collaborating on ECMA work. Thus ECMAScript 4 was
            mothballed.
          </p>
          <h3>Growth and standardization</h3>
          <p>
            During the period of Internet Explorer dominance in the early 2000s,
            client-side scripting was stagnant. This started to change in 2004,
            when the successor of Netscape, Mozilla, released the Firefox
            browser. Firefox was well-received by many, taking significant
            market share from Internet Explorer.
          </p>
          <p>
            In 2005, Mozilla joined ECMA International, and work started on the
            ECMAScript for XML (E4X) standard. This led to Mozilla working
            jointly with Macromedia (later acquired by Adobe Systems), who were
            implementing E4X in their ActionScript 3 language, which was based
            on an ECMAScript 4 draft. The goal became standardizing ActionScript
            3 as the new ECMAScript 4. To this end, Adobe Systems released the
            Tamarin implementation as an open source project. However, Tamarin
            and ActionScript 3 were too different from established client-side
            scripting, and without cooperation from Microsoft, ECMAScript 4
            never reached fruition.
          </p>
          <p>
            Meanwhile, very important developments were occurring in open source
            communities not affiliated with ECMA work. In 2005, Jesse James
            Garrett released a white paper in which he coined the term Ajax and
            described a set of technologies, of which JavaScript was the
            backbone, to create web applications where data can be loaded in the
            background, avoiding the need for full page reloads. This sparked a
            renaissance period of JavaScript, spearheaded by open source
            libraries and the communities that formed around them. Many new
            libraries were created, including jQuery, Prototype, Dojo Toolkit,
            and MooTools.
          </p>
          <p>
            Google debuted its Chrome browser in 2008, with the V8 JavaScript
            engine that was the first to use just-in-time compilation,
            significantly improving execution times. Other browser vendors
            needed to overhaul their engines to compete.
          </p>
          <p>
            In July 2008, these disparate parties came together for a conference
            in Oslo. This led to the eventual agreement in early 2009 to combine
            all relevant work and drive the language forward. The result was the
            ECMAScript 5 standard, released in December 2009.
          </p>
          <h3>Reaching maturity</h3>
          <p>
            Ambitious work on the language continued for several years,
            culminating in an extensive collection of additions and refinements
            being formalized with the publication of ECMAScript 6 in 2015.
          </p>
          <p>
            From 2016 to 2019, a new version of the ECMAScript standard was
            published each year, but the scope of changes was much smaller than
            the 5th or 6th editions. Thus JavaScript can now be considered a
            mature language that has largely settled down.
          </p>
          <p>
            The current JavaScript ecosystem has many libraries and frameworks,
            established programming practices, and increased usage of JavaScript
            outside of web browsers. Plus, with the rise of single-page
            applications and other JavaScript-heavy websites, a number of
            transpilers have been created to aid the development process.
          </p>
        </div>
      </div>
      <div>
        <div>
          <div>
            <h2>Trademark</h2>
            <p>
              "JavaScript" is a trademark of Oracle Corporation in the United
              States. It is used under license for technology invented and
              implemented by Netscape Communications and other parties.
            </p>
            <h2>Website client-side usage</h2>
            <p>
              JavaScript is the dominant client-side scripting language of the
              Web, with 95% of websites using it for this purpose. Scripts are
              embedded in or included from HTML documents and interact with the
              DOM. All major web browsers have a built-in JavaScript engine that
              executes the code on the user's device.
            </p>
          </div>
        </div>
      </div>
      <div>
        <div>
          <h3>Examples of scripted behavior</h3>
          <ul>
            <li>
              Loading new page content without reloading the page. For example,
              social media websites use Ajax so that users can post new messages
              without leaving the page.
            </li>
            <li>
              Animation of page elements, such as fading them in and out,
              resizing, and moving them.
            </li>
            <li>Interactive content, such as games and video.</li>
            <li>
              Validating input values of a web form to make sure that they are
              acceptable before being submitted to the server.
            </li>
            <li>
              Transmitting information about the user's behavior for analytics,
              ad tracking, and personalization.
            </li>
          </ul>
          <h3>Libraries and frameworks</h3>
          <p>
            The majority of websites use a third-party JavaScript library or web
            application framework as part of their client-side page scripting.
          </p>
          <p>
            jQuery is the most popular library, used by over 70% of websites.
          </p>
          <p>
            The Angular framework was created by Google for its web services; it
            is now open source and used by other websites. Likewise, Facebook
            created the React framework for its website and later released it as
            open source; other sites, including Twitter, now use it. There are
            other open source frameworks in use, such as Backbone.js and Vue.js.
          </p>
          <p>
            In contrast, the term "Vanilla JS" has been coined for websites not
            using any libraries or frameworks, instead relying entirely on
            standard JavaScript functionality.
          </p>
          <h2>Other usage</h2>
          <p>
            The use of JavaScript has expanded beyond its web browser roots.
            JavaScript engines are now embedded in a variety of other software
            systems, both for server-side website deployments and non-browser
            applications.
          </p>
          <p>
            Initial attempts at promoting server-side JavaScript usage were
            Netscape Enterprise Server and Microsoft's Internet Information
            Services, but they were small niches. Server-side usage eventually
            started to grow in the late-2000s, with the creation of Node.js and
            other approaches.
          </p>
          <p>
            Electron, Cordova, and other software frameworks have been used to
            create many applications with behavior implemented in JavaScript.
            Other non-browser applications include Adobe Acrobat support for
            scripting PDF documents and GNOME Shell extensions written in
            JavaScript.
          </p>
        </div>
      </div>
      <div>
        <div>
          <div>
            <p>
              JavaScript has recently begun to appear in some embedded systems,
              usually by leveraging Node.js.
            </p>
            <h2>Features</h2>
            <p>
              The following features are common to all conforming ECMAScript
              implementations, unless explicitly specified otherwise.
            </p>
          </div>
        </div>
      </div>
      <h3>Imperative and structured</h3>
      <p>
        JavaScript supports much of the structured programming syntax from C
        (e.g., if statements, while loops, switch statements, do while loops,
        etc.). One partial exception is scoping: JavaScript originally had only
        function scoping with var. ECMAScript 2015 added keywords let and const
        for block scoping, meaning JavaScript now has both function and block
        scoping. Like C, JavaScript makes a distinction between expressions and
        statements. One syntactic difference from C is automatic semicolon
        insertion, which allows the semicolons that would normally terminate
        statements to be omitted.
      </p>
      <h3>Weakly typed</h3>
      <p>
        JavaScript is weakly typed, which means certain types are implicitly
        cast depending on the operation used.
      </p>
      <ul>
        <li>
          The binary + operator casts both operands to a string unless both
          operands are numbers. This is because the addition operator doubles as
          a concatenation operator
        </li>
        <li>The binary - operator always casts both operands to a number</li>
        <li>Both unary operators (+, -) always cast the operand to a number</li>
      </ul>
      <p>
        Values are casted to numbers by casting to strings and then casting the
        strings to numbers. These processes can be modified by defining toString
        and valueOf functions on the prototype for string and number casting
        respectively.
      </p>
      <p>
        JavaScript has received criticism for the way it implements these
        conversions as the complexity of the rules can be mistaken for
        inconsistency. For example, when adding a number to a string, the number
        will be cast to a string before performing concatenation, but when
        subtracting a number from a string, the string is cast to a number
        before performing subtraction.
      </p>
      <p>
        Often also mentioned is {} + [] resulting in 0 (number). This is
        misleading: the {} is interpreted as an empty code block instead of an
        empty object, and the empty array is cast to a number by the remaining
        unary + operator. If you wrap the expression in parentheses ({} + [])
        the curly brackets are interpreted as an empty object and the result of
        the expression is "[object Object]" as expected.
      </p>
      <h3>Dynamic</h3>
      <h4>Typing</h4>
      <p>
        JavaScript is dynamically typed like most other scripting languages. A
        type is associated with a value rather than an expression. For example,
        a variable initially bound to a number may be reassigned to a string.
        JavaScript supports various ways to test the type of objects, including
        duck typing.
      </p>
      <h4>Run-time evaluation</h4>
      <p>
        JavaScript includes an eval function that can execute statements
        provided as strings at run-time.
      </p>
      <h3>Object-orientation (prototype-based)</h3>
      <p>
        In JavaScript, an object is an associative array, augmented with a
        prototype (see below); each string key provides the name for an object
        property, and there are two syntactical ways to specify such a name: dot
        notation (obj.x = 10) and bracket notation (obj['x'] = 10). A property
        may be added, rebound, or deleted at run-time. Most properties of an
        object (and any property that belongs to an object's prototype
        inheritance chain) can be enumerated using a for...in loop.
      </p>
      <h4>Prototypes</h4>
      <p>
        JavaScript uses prototypes where many other object-oriented languages
        use classes for inheritance. It is possible to simulate many class-based
        features with prototypes in JavaScript.
      </p>
      <h4>Functions as object constructors</h4>
      <p>
        Functions double as object constructors, along with their typical role.
        Prefixing a function call with new will create an instance of a
        prototype, inheriting properties and methods from the constructor
        (including properties from the Object prototype). ECMAScript 5 offers
        the Object.create method, allowing explicit creation of an instance
        without automatically inheriting from the Object prototype (older
        environments can assign the prototype to null). The constructor's
        prototype property determines the object used for the new object's
        internal prototype. New methods can be added by modifying the prototype
        of the function used as a constructor. JavaScript's built-in
        constructors, such as Array or Object, also have prototypes that can be
        modified. While it is possible to modify the Object prototype, it is
        generally considered bad practice because most objects in JavaScript
        will inherit methods and properties from the Object prototype, and they
        may not expect the prototype to be modified.
      </p>
      <h4>Functions as methods</h4>
      <p>
        Unlike many object-oriented languages, there is no distinction between a
        function definition and a method definition. Rather, the distinction
        occurs during function calling; when a function is called as a method of
        an object, the function's local this keyword is bound to that object for
        that invocation.
      </p>
      <h3>Functional</h3>
      <p>
        A function is first-class; a function is considered to be an object. As
        such, a function may have properties and methods, such as .call() and
        .bind(). A nested function is a function defined within another
        function. It is created each time the outer function is invoked. In
        addition, each nested function forms a lexical closure: The lexical
        scope of the outer function (including any constant, local variable, or
        argument value) becomes part of the internal state of each inner
        function object, even after execution of the outer function concludes.
        JavaScript also supports anonymous functions.
      </p>
      <h3>Delegative</h3>
      <p>JavaScript supports implicit and explicit delegation.</p>
      <h4>Functions as roles (Traits and Mixins)</h4>
      <p>
        JavaScript natively supports various function-based implementations of
        Role patterns like Traits and Mixins. Such a function defines additional
        behavior by at least one method bound to the this keyword within its
        function body. A Role then has to be delegated explicitly via call or
        apply to objects that need to feature additional behavior that is not
        shared via the prototype chain.
      </p>
      <h4>Object composition and inheritance</h4>
      <p>
        Whereas explicit function-based delegation does cover composition in
        JavaScript, implicit delegation already happens every time the prototype
        chain is walked in order to, e.g., find a method that might be related
        to but is not directly owned by an object. Once the method is found it
        gets called within this object's context. Thus inheritance in JavaScript
        is covered by a delegation automatism that is bound to the prototype
        property of constructor functions.
      </p>
      <h3>Miscellaneous</h3>
      <h4>Run-time environment</h4>
      <p>
        JavaScript typically relies on a run-time environment (e.g., a Web
        browser) to provide objects and methods by which scripts can interact
        with the environment (e.g., a webpage DOM). It also relies on the
        run-time environment to provide the ability to include/import scripts
        (e.g., HTML script elements. This is not a language feature per se, but
        it is common in most JavaScript implementations. JavaScript processes
        messages from a queue one at a time. JavaScript calls a function
        associated with each new message, creating a call stack frame with the
        function's arguments and local variables. The call stack shrinks and
        grows based on the function's needs. When the call stack is empty upon
        function completion, JavaScript proceeds to the next message in the
        queue. This is called the event loop, described as "run to completion"
        because each message is fully processed before the next message is
        considered. However, the language's concurrency model describes the
        event loop as non-blocking: program input/output is performed using
        events and callback functions. This means, for instance, that JavaScript
        can process a mouse click while waiting for a database query to return
        information.
      </p>
      <h4>Variadic functions</h4>
      <p>
        An indefinite number of parameters can be passed to a function. The
        function can access them through formal parameters and also through the
        local arguments object. Variadic functions can also be created by using
        the bind method.
      </p>
      <h4>Array and object literals</h4>
      <p>
        Like many scripting languages, arrays and objects (associative arrays in
        other languages) can each be created with a succinct shortcut syntax. In
        fact, these literals form the basis of the JSON data format.
      </p>
      <h4>Regular expressions</h4>
      <p>
        JavaScript also supports regular expressions in a manner similar to
        Perl, which provide a concise and powerful syntax for text manipulation
        that is more sophisticated than the built-in string functions.
      </p>
      <h4>Promises</h4>
      <p>
        JavaScript also supports promises, which are a way of handling
        asynchronous operations. There is a built-in Promise object that gives
        access to a lot of functionalities for handling promises, and defines
        how they should be handled. It allows one to associate handlers with an
        asynchronous action's eventual success value or failure reason. This
        lets asynchronous methods return values like synchronous methods:
        instead of immediately returning the final value, the asynchronous
        method returns a promise to supply the value at some point in the
        future. Recently, combinator methods were introduced in the JavaScript
        specification, which allows developers to combine multiple JavaScript
        promises and do operations on the basis of different scenarios. The
        methods introduced are: Promise.race, Promise.all, Promise.allSettled
        and Promise.any.
      </p>
      <h3>Vendor-specific extensions</h3>
      <p>
        Historically, some JavaScript engines supported these non-standard
        features:
      </p>
      <ul>
        <li>conditional catch clauses (like Java)</li>
        <li>array comprehensions and generator expressions (like Python)</li>
        <li>
          concise function expressions (function(args) expr; this experimental
          syntax predated arrow functions)
        </li>
        <li>
          ECMAScript for XML (E4X), an extension that adds native XML support to
          ECMAScript (unsupported in Firefox since version 21)
        </li>
      </ul>
      <h2>Security</h2>
      <p>
        JavaScript and the DOM provide the potential for malicious authors to
        deliver scripts to run on a client computer via the Web. Browser authors
        minimize this risk using two restrictions. First, scripts run in a
        sandbox in which they can only perform Web-related actions, not
        general-purpose programming tasks like creating files. Second, scripts
        are constrained by the same-origin policy: scripts from one Web site do
        not have access to information such as usernames, passwords, or cookies
        sent to another site. Most JavaScript-related security bugs are breaches
        of either the same origin policy or the sandbox.
      </p>
      <h3>Cross-site vulnerabilities</h3>
      <p>
        A common JavaScript-related security problem is cross-site scripting
        (XSS), a violation of the same-origin policy. XSS vulnerabilities occur
        when an attacker is able to cause a target Web site, such as an online
        banking website, to include a malicious script in the webpage presented
        to a victim. The script in this example can then access the banking
        application with the privileges of the victim, potentially disclosing
        secret information or transferring money without the victim's
        authorization. A solution to XSS vulnerabilities is to use HTML escaping
        whenever displaying untrusted data.
      </p>
      <p>
        Some browsers include partial protection against reflected XSS attacks,
        in which the attacker provides a URL including malicious script.
        However, even users of those browsers are vulnerable to other XSS
        attacks, such as those where the malicious code is stored in a database.
        Only correct design of Web applications on the server side can fully
        prevent XSS.
      </p>
      <p>
        Another cross-site vulnerability is cross-site request forgery (CSRF).
        In CSRF, code on an attacker's site tricks the victim's browser into
        taking actions the user did not intend at a target site (like
        transferring money at a bank). When target sites rely solely on cookies
        for request authentication, requests originating from code on the
        attacker's site can carry the same valid login credentials of the
        initiating user. In general, the solution to CSRF is to require an
        authentication value in a hidden form field, and not only in the
        cookies, to authenticate any request that might have lasting effects.
        Checking the HTTP Referrer header can also help.
      </p>
      <h3>Misplaced trust in the client</h3>
      <p>
        Developers of client-server applications must recognize that untrusted
        clients may be under the control of attackers. The application author
        cannot assume that their JavaScript code will run as intended (or at
        all) because any secret embedded in the code could be extracted by a
        determined adversary.
      </p>
      <h3>Misplaced trust in developers</h3>
      <p>
        Package management systems such as npm and Bower are popular with
        JavaScript developers. Such systems allow a developer to easily manage
        their program's dependencies upon other developer's program libraries.
        Developers trust that the maintainers of the libraries will keep them
        secure and up to date, but that is not always the case. A vulnerability
        has emerged because of this blind trust. Relied-upon libraries can have
        new releases that cause bugs or vulnerabilities to appear in all
        programs that rely upon the libraries. Inversely, a library can go
        unpatched with known vulnerabilities out in the wild. In a study done
        looking over a sample of 133k websites, researchers found 37% of the
        websites included a library with at least one known vulnerability. "The
        median lag between the oldest library version used on each website and
        the newest available version of that library is 1,177 days in ALEXA, and
        development of some libraries still in active use ceased years ago."
        Another possibility is that the maintainer of a library may remove the
        library entirely. This occurred in March 2016 when Azer Koçulu removed
        his repository from npm. This caused all tens of thousands of programs
        and websites depending upon his libraries to break.
      </p>
      <h3>Browser and plugin coding errors</h3>
      <p>
        JavaScript provides an interface to a wide range of browser
        capabilities, some of which may have flaws such as buffer overflows.
        These flaws can allow attackers to write scripts that would run any code
        they wish on the user's system. This code is not by any means limited to
        another JavaScript application. For example, a buffer overrun exploit
        can allow an attacker to gain access to the operating system's API with
        superuser privileges.
      </p>
      <p>
        Plugins, such as video players, Adobe Flash, and the wide range of
        ActiveX controls enabled by default in Microsoft Internet Explorer, may
        also have flaws exploitable via JavaScript (such flaws have been
        exploited in the past).
      </p>
      <p>
        In Windows Vista, Microsoft has attempted to contain the risks of bugs
        such as buffer overflows by running the Internet Explorer process with
        limited privileges. Google Chrome similarly confines its page renderers
        to their own "sandbox".
      </p>
      <h3>Sandbox implementation errors</h3>
      <p>
        Web browsers are capable of running JavaScript outside the sandbox, with
        the privileges necessary to, for example, create or delete files. Such
        privileges are not intended to be granted to code from the Web.
      </p>
      <p>
        Incorrectly granting privileges to JavaScript from the Web has played a
        role in vulnerabilities in both Internet Explorer and Firefox. In
        Windows XP Service Pack 2, Microsoft demoted JScript's privileges in
        Internet Explorer.
      </p>
      <p>
        Microsoft Windows allows JavaScript source files on a computer's hard
        drive to be launched as general-purpose, non-sandboxed programs (see:
        Windows Script Host). This makes JavaScript (like VBScript) a
        theoretically viable vector for a Trojan horse, although JavaScript
        Trojan horses are uncommon in practice.[failed verification]
      </p>
    </div>
    <side-catalog class="catalog" v-bind="catalogProps">
      <!-- <template #default="{level, isActive}"> -->
      <!-- <i :class="['line-style', isActive ? 'line-style--active' : '']"></i> -->
      <!-- <i :class="['iconfont', level<=2 ? isActive ? 'icon-smile' : 'icon-normal': '']"></i>
      </template> -->
      <!-- <template #row="{level, isActive, title}">
        * <span :style="{'margin-left': level*15+'px'}">{{ title }}</span>
      </template> -->
    </side-catalog>
  </div>
</template>

<script>
import SideCatalog from "./components/main.vue";
export default {
  name: "app",
  components: {
    SideCatalog,
  },
  data() {
    return {
      vhtml: "",
      catalogProps: {
        // 内容容器selector(必需)
        container: "#demo",
        height: "calc(100% - 100px)",
        title: "Contents",
        levelList: ["h2", "h3", "h4"],
      },
    };
  },
};
</script>

<style scoped>
#app {
  /* display: flex; */
}
#demo {
  width: 700px;
  overflow: auto;
  padding: 20px;
  line-height: 1.5;
  /* height: 300px; */
  border: 1px solid;
  margin-right: 50px;
  /* margin-top: 100px; */
}
p {
  text-indent: 2em;
}
.catalog {
  position: fixed;
  top: 50px;
  right: 50px;
}
.iconfont {
  width: 16px;
}

.line-style {
  display: inline-block;
  height: 20px;
  width: 3px;
  background: transparent;
}
.line-style--active {
  background: currentColor;
  /* color: currentColor; */
}
</style>
